In a wireless communications technology, a MIMO (Multiple-Input Multiple-Output, multiple-input multiple-output) system supports multiple-input multiple-output transmission from multiple (M) transmit antennas to multiple (N) receive antennas. Certainly, the MIMO system is also compatible with SISO (Single-Input Single-Output, single-input single-output) transmission and the like.
In the MIMO system, channel estimation usually needs to be performed on a radio channel between a base station and a terminal to restore data transmitted through the radio channel. Generally, the base station transmits a pilot signal, and the terminal receives the pilot signal to perform channel estimation. The pilot signal includes a pilot signal sequence that is pre-known by both the base station and the terminal. Therefore, the terminal can perform channel estimation based on the received pilot signal and the pre-known pilot signal sequence.
In the MIMO system, a result of performing channel estimation by using a pilot signal has two functions: One is to perform channel sounding, so that the terminal performs channel state information (Channel State Information, CSI) estimation, including CQI (Channel quality indicator, channel quality indicator) estimation, rank information estimation, PCI (Pre-Coding Indication, pre-coding indication) information estimation, and the like. After completing the CSI estimation, the terminal feeds back CSI information to the base station through an uplink feedback channel. The other is to perform channel estimation for data demodulation.
A four-branch multiple-input multiple-output system 4 Branch MIMO system that is newly introduced into the 3GPP protocol, that is, a 4×4 (4-input 4-output) MIMO system, a 4×2 MIMO system, or a 4×1 MIMO system, is used as an example. Because rank selection, PCI selection, and channel quality estimation for a 4×4 MIMO channel need to be supported and demodulation for a maximum of 4 streams also needs to be supported, a pilot signal needs to be transmitted on all four antennas of a base station, so as to support 4 Branch MIMO terminals (including a 4×4 MIMO terminal, a 4×2 MIMO terminal, and a 4×1 MIMO terminal) to perform channel estimation. Further, the terminal may perform CSI estimation and data demodulation.
Therefore, in the MIMO system (such as a 4 Branch MIMO system or an 8 Branch MIMO system), on a pre-condition that an existing sending mode of 2×2 MIMO is still in use for a first antenna and a second antenna, how to reduce interference, which is caused by a pilot signal transmitted on a newly added antenna to a legacy terminal (that is, an existing SISO UE and an existing 2 Branch MIMO UE), and ensure necessary receiving performance of a MIMO terminal become an important factor that needs to be considered in pilot signal design of the MIMO system. In addition, in the MIMO system, a downlink code channel resource is very valuable, and therefore, how to reduce consumption of a code channel resource is also a factor that further needs to be considered.